x86, mm: Fix page table early allocation offset checking

During debugging loading kernel above 4G, found that one page is not used
in pre-allocated BRK area for early page allocation.
pgt_buf_top is address that can not be used, so should check if that new
end is above that top, otherwise last page will not be used.

Fix that checking and also add print out for allocation from pre-allocated
BRK area to catch possible bugs later.

But after we get back that page for pgt, it tiggers one bug in pgt allocation
with xen: We need to avoid to use page as pgt to map range that is
overlapping with that pgt page.

Add checking about overlapping, when it happens, use memblock allocation
instead.  That fixes crash on Xen PV guest with 2G that Stefan found.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1359058816-7615-2-git-send-email-yinghai@kernel.org
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Tested-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>